Over-detection or spontaneous regression of breast cancer?
Dear Editor,
The results of the interesting model study by Barratt et al. [1]
suggest that more breast cancer are diagnosed among screened than
unscreened women. For women aged 50 who have five biennial mammography
screenings, 40% higher cumulative 10-year cumulative incidence of breast
cancer was observed (and 67% higher if ductal carcinoma in-situ (DCIS) was
included). For women screened from other ages, the model predicted a
similar relative increase over ten year.
Two important underlying assumptions in mammography screening are i)
the sensitivity at the screening is relatively high (>75%) and ii) most
invasive tumours grow monotonously; i.e. spontaneous tumour regression is
uncommon.
From these assumptions it emerges that most of the difference in the
cumulative incidences between the screened and the unscreened women should
disappear if a prevalence screening of the previously unscreened women had
been performed at the end of the 10-year period.
However, a prevalence screening can only compensate for a part of
this difference. The reason for this is that the detection rate at a
prevalence screening repeatedly has been shown to be only about 3 times
higher than the background incidence at age 60 or 70 [2].
Moreover, the model by Barratt et al. predicts that if 1000 women
aged 40 have 15 biennial mammography screenings from age 40 to 69, the
cumulative difference between screened and unscreened for the three 10-
year periods is (17.6 – 13.2) + (28.1 – 19.8) + (32.5 – 23.9) = 21.3
invasive breast cancers [1]. This calculation leads to the conclusion that
the model predicts the detection rate at a prevalence screening of women
aged 70 to be twice that observed in screening programmes [3, 4].
This result suggests that at least one of the two assumptions above
should be modified. The prevalence screening detects most of the slow-
growing cancers, and the detection rate at the following screenings is
stable at a lower level. This indicates that it is true that the
sensitivity at the screening is relatively high.
We conclude that the other assumption is false. It is not true that
spontaneous tumour regression is uncommon.
1. Barrratt A, Howard K, Irwig L, Salkeld G, Houssami N. Model of
outcomes of screening: information to support informed choices. Br Med J
doi:10.1136/bmj.38398.469479.8F.
2. Zahl PH, Andersen JM, Maehlen, J. Spontaneous regression of
cancerous tumours detected by mammography screening. JAMA 2004; 292: 2579-
80.
3. Tabár L, Fagerberg G, Duffy SW, Day NE, Gad A, Gröntoft O. Update
of the Swedish Two-County program of mammographic screening for breast
cancer. Radiol Clin North Am 1992; 30: 187-210.
4. Smith-Bindman R, Chu PW, Miglioretti Dl, Sickles EA, Blanks R, et
al. Comparison of screening mammography in the United States and the
United Kingdom. JAMA 2003; 290: 2129-38.
Competing interests:
None declared
Competing interests:
No competing interests
11 March 2005
Per-Henrik Zahl
Senior Statistician
Professor Jan Mæhlen
Norwegian Institute of Public Health, PO Box 4404 Nydalen, N-0403 Oslo, Norway
Rapid Response:
Over-detection or spontaneous regression of breast cancer?
Dear Editor,
The results of the interesting model study by Barratt et al. [1]
suggest that more breast cancer are diagnosed among screened than
unscreened women. For women aged 50 who have five biennial mammography
screenings, 40% higher cumulative 10-year cumulative incidence of breast
cancer was observed (and 67% higher if ductal carcinoma in-situ (DCIS) was
included). For women screened from other ages, the model predicted a
similar relative increase over ten year.
Two important underlying assumptions in mammography screening are i)
the sensitivity at the screening is relatively high (>75%) and ii) most
invasive tumours grow monotonously; i.e. spontaneous tumour regression is
uncommon.
From these assumptions it emerges that most of the difference in the
cumulative incidences between the screened and the unscreened women should
disappear if a prevalence screening of the previously unscreened women had
been performed at the end of the 10-year period.
However, a prevalence screening can only compensate for a part of
this difference. The reason for this is that the detection rate at a
prevalence screening repeatedly has been shown to be only about 3 times
higher than the background incidence at age 60 or 70 [2].
Moreover, the model by Barratt et al. predicts that if 1000 women
aged 40 have 15 biennial mammography screenings from age 40 to 69, the
cumulative difference between screened and unscreened for the three 10-
year periods is (17.6 – 13.2) + (28.1 – 19.8) + (32.5 – 23.9) = 21.3
invasive breast cancers [1]. This calculation leads to the conclusion that
the model predicts the detection rate at a prevalence screening of women
aged 70 to be twice that observed in screening programmes [3, 4].
This result suggests that at least one of the two assumptions above
should be modified. The prevalence screening detects most of the slow-
growing cancers, and the detection rate at the following screenings is
stable at a lower level. This indicates that it is true that the
sensitivity at the screening is relatively high.
We conclude that the other assumption is false. It is not true that
spontaneous tumour regression is uncommon.
1. Barrratt A, Howard K, Irwig L, Salkeld G, Houssami N. Model of
outcomes of screening: information to support informed choices. Br Med J
doi:10.1136/bmj.38398.469479.8F.
2. Zahl PH, Andersen JM, Maehlen, J. Spontaneous regression of
cancerous tumours detected by mammography screening. JAMA 2004; 292: 2579-
80.
3. Tabár L, Fagerberg G, Duffy SW, Day NE, Gad A, Gröntoft O. Update
of the Swedish Two-County program of mammographic screening for breast
cancer. Radiol Clin North Am 1992; 30: 187-210.
4. Smith-Bindman R, Chu PW, Miglioretti Dl, Sickles EA, Blanks R, et
al. Comparison of screening mammography in the United States and the
United Kingdom. JAMA 2003; 290: 2129-38.
Competing interests:
None declared
Competing interests: No competing interests